The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During combustion in a diesel engine, an air/fuel mixture is delivered through an intake valve to cylinders and is compressed and combusted therein. After combustion, the piston forces the exhaust gas (i.e., the exhaust stream) to flow from the cylinders through an exhaust system, from which the exhaust stream is released to the atmosphere. The exhaust stream may contain oxides of nitrogen (NOx) and carbon monoxide (CO).
Exhaust stream treatment systems may employ catalysts in one or more components configured for accomplishing an SCR process such as reducing nitrogen oxides (NOx) to produce more tolerable exhaust constituents of nitrogen (N2) and water (H2O). Reductant may be added to the exhaust stream upstream from an SCR, and, for example only, the reductant may include anhydrous ammonia (NH3), aqueous ammonia or urea, any or all of which may be injected as a fine mist into the exhaust stream. When the ammonia, mixed with the other constituents of the exhaust stream, reaches the SCR component, the NOx emissions within the exhaust stream are broken down. A Diesel Particulate Filter (DPF) may then capture soot, and that soot may be periodically incinerated during regeneration cycles. Water vapor, nitrogen and reduced emissions exit the exhaust system.
To maintain efficient NOx reduction in the SCR component, a control may be employed so as to maintain a desired quantity of the reductant (i.e., reductant load) in the SCR component. As the exhaust stream, containing NOx, passes through the SCR component, the reductant is consumed, and the load is depleted. A model may be employed by the control to track and/or predict how much reductant is loaded in the SCR component and to maintain an appropriate reductant load for achieving a desired effect such as reduction of NOx in the exhaust stream.
Exhaust systems with SCR components may be vulnerable to poor quality reductant. If a reductant tank has been filled with poor quality reductant, an exhaust diagnostic system may detect an unacceptable level of performance for the SCR component, such as a low NOx reduction efficiency. In response to detection of such a condition, the engine control modules in some vehicles may impose limits on the speed of the vehicle and/or initiate other remedial actions. For example, if acceptable reductant is not added soon after detection of a low SCR conversion efficiency, some controls may limit vehicle speed, e.g., to 55 mph and ultimately to 4 mph, in accordance with government requirements. To avoid imposition of these or other measures, a supply of high quality reductant should be maintained, and, if poor quality reductant is detected, it should promptly be replaced with higher quality reductant.
Testing the SCR efficiency is usually performed at SCR temperatures such as, for example, at 250 degrees C. or hotter. During speed limitation, however, the temperature range of the exhaust stream may be, for example only, less than 250 degrees C. Thus, after a vehicle has been speed limited and/or other remedial action has been taken, the remedial measures may prevent sufficient heat from being generated in the exhaust stream to accurately evaluate the SCR conversion efficiency. In this way, the imposition of remedial measures may interfere with the ability to verify whether the reductant has been replaced. Unfortunately, without an evaluation of the SCR conversion efficiency, no acceptable method currently exists to reset the exhaust diagnostic system after the speed of the vehicle has been limited following detection of poor reductant quality and/or low SCR conversion efficiency.
Some have attempted to avoid this problem by resetting the exhaust diagnostic system using a scan tool, e.g., at a service shop. This solution is unfavorable, however, due to the ready availability of scan tools to the general public, enabling some vehicle operators to inappropriately reset the exhaust diagnostic system to circumvent the purpose behind the measures and thereby enable use of ineffective reductants such as water. As a result, in some cases, no mechanism is provided for resetting the system via a scan tool. Moreover, many customers lack access to a scan tool. For these customers, a visit to an authorized service shop may be required anyway to enable the service shop to perform a service test to determine whether reductant of suitable quality has been replenished. Nonetheless, even where reductant quality can reliably be determined, when a vehicle has encountered a NOx efficiency issue, the model employed by the control to track and/or predict how much reductant is loaded in the SCR component may be susceptible to inaccuracies.
Accordingly, it is desirable to provide a system and method for predicting a quantity of reductant (i.e., the reductant load) present on SCR components and for testing the efficiency at which NOx are reduced in such SCR components with improved reliability following detection of poor reductant quality and/or low SCR conversion efficiency.